1. Field of the Invention
The present invention relates to an optically clear, plastic member useful as a video disc structure, a composition of material for use in making such optically clear, plastic member, and a method for molding a video disc member from this composition of material, and more particularly, to a video disc structure having increased structural flexibility, improved resistance to breakage and a minimum and uniform value of birefringence extending across the entire surface of the disc.
An object of the present invention is to provide an optically clear, plastic member useful as a protective, transmissive layer for an impinging light beam.
Another object of the present invention is to provide a video disc structure having a uniform value of birefringence extending across the entire surface of the video disc member.
A further object of the present invention is to provide a video disc structure being improved flexibility and resistance both to breakage and the formation of internal stress lines due to expected handling.
2. Description of the Prior Art
U.S. Pat. No. 4,124,672 to Manfred H. Jarsen, entitled "Replication Utilizing A Casting Process", describes a video disc member which includes a body member including integrally formed information indicia, a reflective coating covering the information indicia, and a protective coating formed over the reflective layer.
The information indicia are further described as light reflective and light non-reflective elements positioned sequentially in track-like fashion on a surface of the video disc member. The light reflective and light non-reflective elements are arranged to represent the frequency modulated signal. The frequency band width extends from 0.25 Mz to 10 Mz. In the preferred embodiment, such a video disc member is employed for storing a frequency modulated signal representing a composite video signal. The composite video signal contains both a chrominance signal in combination with the luminance signal.
In one mode of operation of such a video disc, the read beam passed through the protective layer and is modulated by the information indicia. The reflective layer enhances both the light reflectivity of the light reflective regions and the light scattering of the light non-reflective regions.
In an alternate mode of operation, the read beam impinges upon the undersurface of the video disc substrate member and passes through the substrate body. After passing through the substrate body, the read beam impinges upon the information indicia and is modulated by the information indicia carried thereon. In this second mode of operation, the reflective layer also enhances the reflection from the light reflective regions and also enhances the scattering from the light non-reflective regions.
In such a video disc structure, a number of areas of improvement are herein identified for improving the quality of a video picture recovered from the video disc and displayed upon a standard television receiver.
A first such area of improvement is related to improving the radial error tracking signal by providing improved clarity and birefringence in the thick video disc body through which the reading beam passes prior to impinging upon the information indicia carried thereby. Typical video disc members have a thickness falling within the range of 0.101 MM to 2.5 MM. The birefringence of such a substrate body is measured in nanometers. A nanometer is a measure of the optical retardation and distortion of light as it passes through a transparent body.
By following the manufacturers suggested molding time of a two second injection cycle at a suggested temperature of about 480.degree. F. video disc members were formed having commercially non-acceptable characteristics. The video disc members were not flat but rather were umbrella shaped in cross-section. This umbrella shape is measured at the center of the disc by measuring the distance the center of the disc is spaced from a plane passing through at least two points on the outer periphery of the disc. This distance is identified as the crown of the disc. In the example given above, this crown is 10 MM.
A further non-acceptable characteristic is the lack of uniform values of birefringence exhibited by the information storage area of such a video disc. Values of birefringence ranging from a low of four nanometers to a high of one hundred nanometers were measured. A substantially uniform value of birefringence of plus or minus five nanometers over the entire information storage area is preferred.
Such a prior art disc is brittle and subject to easy surface scratching and the generation of internal stress lines which result in surface cracks.
A final non-acceptable characteristic for a video disc member is the presence of splay marks, blisters and other surface imperfections. All of these characteristics affect the quality of the modulated light beam recovered after reflection from the video disc surface.